Light causes changes in the photoreceptor units of the retina which ultimately results in a visual response. From our work and work from other laboratories, it seems probable that small intracellular messengers called cyclic nucleotides, especially cyclic GMP, are involved in this process. We have found that light affects several enzymes of cyclic GMP metabolism including cyclic GMP phosphodiesterase and guanylate cyclase. The photoreceptor units appear to be unique in the body as they contain exceptionally high amounts of cyclic GMP. Perhaps more importantly, we have now demonstrated a novel protein-kinase activity in rod outer segments which is activated by light and preferentially uses GTP rather than ATP as a phosphate donor. The enzyme specifically phosphorylates rhodopsin. Its activity is both lower in the dark and higher in the light than that of well-known ATP-kinase. Several other features distinguish GTP-kinase activity from that of ATP-kinase. GTP-kinase activity, for example, is strongly inhibited by cyclic AMP and by calcium, the latter which is known to mimic the electrophysiological effect of light on the retina. Guanine nucleotides (eg. cyclic GMP and GTP) may thus function as chemical intermediates in the visual process.